This invention relates to an image coding system for use in coding a sequence of image signals, such as a television signal or a facsimile signal, into a sequence of coded signals. It is to be noted that each of the image signal sequence and the coded signal sequence is produced in a digital form.
A wide variety of algorithms have been used in a conventional coding system so as to code a sequence of image signals into a sequence of coded signals and to reduce an amount of transmission information carried by the coded signal sequence. In this event, the coded signal sequence has been transmitted to a decoding system through a transmission line or path with a transmission rate kept unchanged. Specifically, such a conventional coding system may be a predictive coding system, an orthogonal coding system, a vector quantization system and may be called a high efficiency coding system because the image signal sequence can be transmitted with a high efficiency.
In order to establish such a high efficiency coding system, a plurality of coding characteristics, such as quantization characteristics, may be switched from one to another by the use of a coding control circuit which carries out a coding control operation in response to the image signal sequence and which selects either one of the coding characteristics. It is known in the art that the coding control operation is of very importance on achieving a high quality of a reproduced image or picture.
In an article contributed to IEEE Transactions on Communications, Vol. COM-32, No. 3, March 1984 (page 280-page 287), by Naoki Mukawa et al, a coding control operation is carried out in an image coding system which comprises a quantizer and a buffer memory. More particularly, the quantizer has a plurality of quantization characteristics as the coding characteristics and quantizes the image signal sequence into a sequence of quantized signals in accordance with a selected one of the quantization characteristics. The quantized signals are successively coded into coded signals and stored in the buffer memory which is occupied by the coded signals. During the coding control operation, an amount of the coded signals is monitored and is fed back to the quantizer. As a result, the quantization characteristics are varied in the quantizer from one to another with reference to the amount of coded signals stored in the buffer memory. Thus, the coding control operation is carried out in a feedback manner in the image coding system.
Herein, it is to be noted that consideration is made about avoiding overflow and underflow of the buffer memory by controlling an amount of information stored in the buffer memory. Such control of the amount of information can be carried out, for example, by making a current one of the quantization characteristics rough and fine when the amount of information becomes large and small in the buffer memory, respectively.
As mentioned above, the current quantization characteristic is controlled with reference to an amount of previous information to vary a distortion of quantization. This means that an amount of information is liable to be excessively increased or decreased and that such an excessive increase or decrease is often repeated in the buffer memory. As a result, an occupied area of the buffer memory is repeatedly drastically decreased or increased. Such repeat of a decrease and increase inevitably brings about alternate appearance of a degraded area and a non-degraded area on a reproduced image and results in serious degradation of an image quality.
On the other hand, a coding control operation of a feed forward type is proposed by A. G. Tescher in National Telecommunications Conference Record, 1978, 19.1.1 under the title of "Rate adaptive communication." In this event, measurement is made about an amount of information to be coded. Such information is subjected to quantization into a sequence of quantized signals so that a predetermined transmission rate can be accomplished. The quantized signal sequence is thereafter coded into a coded signal sequence of the predetermined transmission rate. With this method, stable control is possible because the information to be coded is directly controlled. No degradation of the picture quality occurs due to instability of coding control operation. However, such a coding control operation can not be carried out in real time manner because the predetermined transmission rate can not be achieved without a long delay time.
In Unexamined Japanese Patent Publication No. 99,855/1982, an amount of information is estimated in consideration of electric power of a differential signal between two adjacent frames to produce an estimation value. Either one of coding characteristics is selected by the use of the estimation value. However, such an estimation value is very low in precision of estimation because an amount of information is not directly measured from the information to be coded. This results in instability of operation in a coding system.
In Unexamined Japanese Patent Publication No. 154,987/1987, a coding system is proposed wherein orthogonal transform is carried out so as to transform a sequence of image signals into a sequence of transform coefficients. The transform coefficients are coded into a sequence of coded signals by the use of bit allocation tables which define different coding characteristics. A coding control operation is made so as to change the bit allocation tables from one to another. With this system, it is difficult to change the coding characteristics insofar as the bit allocation tables are not switched from one to another. Therefore, such a coding system is not applied to a system which need not any bit allocation table and which may be, for example, a scene adaptive coder proposed by W. Chen in IEEE Transactions on Communications, Vol. COM-32, No. 3, March 1984.